Next to its structural and trophic roles, the extra cellular matrix (ECM) defines an ideal environment for cell to cell communication and determines all the cellular behaviors, including proliferation, migration, differentiation or apoptosis. The molecular mechanisms controlling these processes are getting better understood. In the nervous system, 3 major families of diffusible or transmembrane signals (netrins, semaphorins and ephrins) ensure these functions during embryonic development. (TESSIER-LAVIGNE and GOODMAN, Science, vol. 274, p: 1123-33, 1996). Among them, the semaphorins define a family of more than 25 members subdivided into 8 classes according to their structural specificities (KOLODKIN et al., Cell, vol. 75, p: 1389-99, 1993) and which can be classified as either secreted or transmembrane semaphorins. The secreted ones are class II (invertebrates), III (vertebrates), and V (viral), whereas the other classes (I, IV and VI-VIII) are transmembrane.
Over the past five years, several studies were designed to elucidate the transduction pathways allowing the signaling of the diverse functions of semaphorins ranging from axon guidance, cell migration, cell differentiation to apoptosis in both physiological and pathological conditions. The current view considers that this functional diversity is due to the formation of a receptor complex, highly dynamic, modulating signal integration by selective recruitment and activation of multiple intracellular pathways leading to actin cytoskeleton remodeling (CASTELLANI and ROUGON, Curr. Opin. Neurobiol., vol. 12, p: 532-41, 2002). All of them have a common domain called the “sema” domain of nearly 500 amino acids with 12-16 cysteine residues, which confers the binding specificity of each semaphorin (RAPER, Curr. Opin. Neurobiol., vol. 10, p: 88-94, 2000). Among these different semaphorins, class III semaphorins induce the collapse of neuronal growth cones, which is why they were initially named collapsins (LUO et al., Cell, vol. 75, p: 217-227, 1993). Sema 3A, the molecule that gives the rest of the family its name, is the most extensively studied, and in all cases it has been described as a repellent factor for axons, from sensory neurons and spinal motoneurons to pyramidal neurons of the cortex (MUELLER, Annu. Rev. Neurosci., vol. 22, p: 351-388, 1999). Strikingly, this semaphorin can exert two different effects in the same cell. This has been demonstrated in cortical neurons in which Sema3A acts as a repellent factor for axons and is a chemoattractant for the dendrites (POLLEUX et al., Nature, vol. 404, p: 567-73, 2000; BAGNARD et al., Development, vol. 125(24), p: 5043-53, 1998). In order to explain this phenomenon, it is necessary to consider the existence of a mechanism ensuring a differential transduction in the two cellular poles. More than a principle of differential transduction, it is necessary to understand the mechanisms controlling the molecular hierarchy and to elucidate the formation of supra-molecular structures ensuring the diversity of the cellular behaviors in response to environmental changes.
Hence, recent works demonstrated the role of the two known members of the neuropilin family, neuropilin-1 (NRP1) and neuropilin-2 (NRP2), as the ligand binding sub units of the receptor complex involved in the transduction cascade of class III semaphorins (for review see Bagnard D. (Editor) Neuropilin: from nervous system to vascular and tumor biology. Landes Bioscience-Kluwer Academic/Plenum Publishers Hardbound, ISBN 0-306-47416-6, Advance in Experimental Medicine and Biology Vol. 515, p: 140, 2002). NRP1 and NRP2 are single spanning transmembrane proteins with an (i) extracellular part, which is important for dimerization (RENZI et al., J. Neurosci., vol. 19, p: 7870-7880, 1999), a transmembrane segment, and a short cytoplasmic domain of about 40 amino acids.
Interestingly, NRP1 and NRP2 possess a short intracellular domain without transduction capacity. A molecular explanation for this observation was given when it was found that neuropilins form complexes with receptors belonging to the plexin family, and that the plexin is the transducing element in neuropilin/plexin complex (RHOM et al., Mech. Dev., vol. 93, p: 95-104, 2000; TAMAGONE et al., Cell, vol. 99, p: 71-80, 1999). Finally, signal transduction by class III semaphorins depends upon complex formation between neuropilins with the plexins.
Nevertheless, complexes with plexins are not the only types of complexes formed by neuropilins.
It was found that neuropilins can also form stable complexes with the adhesion molecules L1-CAM and Nr-CAM (CASTELLANI et al., Neuron, vol. 27, p: 237-249, 2000) and mutations in the extracellular domain of L1 or the complete absence of L1 in gene-targeted mice result in the disruption of Sema 3A signaling leading to guidance errors.
Tyrosine kinase receptors may, therefore, also play a role in neuropilin-associated signaling. Thus, it has been observed that the migration of DEV neuroectodermal progenitor cells is repulsed by Sema 3A, and the presence of both NRP1 and VEGFR-1 is required for the repulsion (Bagnard et al., J Neurosci., vol. 21, p: 3332-41, 2001). This interaction explain the inhibition of sprout formation by VEGF in an in vitro model of angiogenesis with Sema 3A (MIAO et al., J. Cell. Biol., vol. 146, p: 233-242, 1999). It has also been found that neuropilins form complexes with VEGFR-2 (SORER et al., Cell, vol. 92, p: 735-45, 1998) and MET (WINBERG et al., Neuron, vol. 32, p: 53-62, 2001).
Recently, it has also been shown that neuropilins form complexes with integrins, and said complexes are able to promote axon outgrowth (PASTERRAMP et al., Nature, vol. 424, p: 398-405, 2003).
Consequently, the above studies contribute to identify class III semaphorins/neuropilins complexes as a potential target for neurodegenerative conditions and cancer as recently evidence (for a review see GUTTMANN-RAVIV et al., Cancer Letter, 2006; CHEDOTAL et al., Cell Death and Differentiation, 2005). In this context, agents that interfere with the complex formation would clearly have therapeutic potential and/or be useful biological tools.
In this way, GARETH et al. (Journal of Neurochemistry, vol. 92, p: 1180-1190, 2005) have used an algorithm in order to design a peptide antagonist of Sema 3A/NRP1 complex. The authors have identified antagonist peptides in the Sema 3A Ig domain, which is implicated in Sema 3A/NRP1 dimerization, and a NRP1 MAM domain, which mediates the lateral dimerization of the receptor but not the ligand binding. The identified antagonist peptides are able to effectively inhibit the growth cone collapse response stimulated by Sema 3A. Nevertheless, these antagonists, which are not located in the transmembrane domain, have an IC50 of more than 1 μM, said concentration being too important to enable the use of such an antagonist in therapy.
So, there is a recognized and permanent need in the art for new antagonists of class III semaphorins/neuropilins complexes, which can be used in therapies.